Finding razors in "the wild" is IMO one of the best thrills in this sport.
Local antique shops, flea markets, estate sales, Craigslist, garage or rumage sales can keep you occuoied for months in your local area alone.
This sums it up.
Typically you can also get them in antique or estate sale cheaper then ebay or Craigslist. Though this is getting harder the more popular SRs are getting as every antique dealer thinks they have gold now with every beat up broken SR they find.